Magnetic north is slightly shifted from true north, and therefore true north cannot be detected by a magnetic compass. However, administrative maps are produced based on true north, and the Construction Standards Act is also based on true north. Therefore, in the field of civil engineering and construction, true north must be measured correctly. In underground tunnel construction, in particular, a magnetic compass does not function correctly because of the effect of a mineral vein or the like.
A conventional gyro compass that detects the earth's rotational angular velocity to determine true north has been known as a true north detector that detects true north correctly. Gyro compasses are generally orthogonal three-axis type devices. The gyro compasses are large in size and highly costly.
Single-axis or two-axis type gyro compasses have been proposed in order to reduce the size and the cost. Most of such gyro compasses however have a gyro sensor and an acceleration sensor rotated on a rotation base. However, a large space must be secured for its large rotation angle, and this limits how compact the device can be. Most single-axis type gyro compasses need a horizontal plane, which makes it cumbersome to handle them. Single-axis type gyro compasses that do not need a horizontal plane have been suggested. Their direction measuring accuracies are inferior to those of three-axis type compasses.
Therefore, the applicant has suggested a six-direction indicator that is a single-axis type device and does not need a horizontal plane in the disclosure of JP 2012-112679 A. The six-direction indicator takes account of a UVW rectangular coordinate system in addition to an XYZ rectangular coordinate system. The six directions, +U, −U, +V, −V, +W, and −W are provided apart at intervals of 60 deg when they are projected orthogonally on an YZ plane.
The six-direction indicator includes a shaft, a driving source, a rotation member, and a guide member. The shaft extends in the X-direction. The driving source rotates the shaft around the X axis. The rotation member is rotatable around an axis inclined at α deg with respect to the shaft and coupled with the shaft. The rotation member includes a spherical surface and an orbit portion formed around the x-axis of the spherical surface. The guide member is fixed to an XYZ rectangular coordinate system and in contact with the orbit portion. The six-direction indicator takes account of an xyz coordinate system fixed to the rotation member. The xyz coordinate system rotates within the XYZ coordinate system according to the rotation of the rotation member. The orbit portion has such a shape that the y-axis of the rotation member is directed in each of the +U, −U, +V, −V, +W, and −W directions when the rotation member rotates in contact with the guide member by the rotation of the shaft.
An angular velocity sensor and an acceleration sensor are provided in the rotation member. The angular velocity sensor detects an angular velocity around the y-direction. The acceleration sensor detects a gravitational acceleration around the y-direction. The six-direction indicator measures a rotational angular velocity and a gravitational acceleration around the U, V, and W axes by directing the y-axis of the rotation member sequentially in the directions of the +U, −U, +V, −V, +W, and −W. An azimuth angle ψ is calculated based on the measured rotational angular velocity and gravitational acceleration.
It has been known that the angular velocity sensor outputs an angular velocity that is not zero (offset) even though there is no rotation generated around the detection axis. The offset causes a measurement error in the rotational angular velocities around the U, V, and W axes. Therefore, the six-direction indicator for example calculates the difference between a rotational angular velocity measured when the y-axis is directed in the +V direction and a rotational angular velocity measured when the y-axis is directed in the −V direction. In this way, the offsets included in the rotational angular velocities for the +V axis and −V axis are canceled.